Eddy current control in NMR imaging systems

ABSTRACT

A primary field magnet assembly, of the type used in magnetic resonance imaging systems, which has been designed with the capability to suppress eddy currents during magnetic resonance imaging through the use of eddy current suppressing material. In addition, this invention includes the use of high conductivity material to decouple the imaging system radio frequency antennas from their environment, which generally includes the primary field magnet assembly, and can be used to preserve the signal-to-noise performance of the scanner. Such highly conductive material is effective when employed in combination with the primary field magnet assembly described herein.

BACKGROUND OF THE INVENTION

The present invention pertains to the use of magnetic materials for thesuppression of eddy currents, and the use of highly conductive materialsto enhance signal-to-noise performance as related to radio frequencyantennas of magnetic resonance (NMR) imaging systems.

Magnetic resonance imaging techniques in present use generally employpulsed magnetic field gradients to spatially encode the nuclear magneticresonance (NMR)signal from various portions of an anatomical region ofinterest. The pulsed magnetic field gradients together with radiofrequency excitation of the nuclear spins, and acquisition of signalinformation are commonly referred to as a pulse sequence.

Pulsing current through a set of conductors will produce a magneticfield external to the conductor; the magnetic field generally has thesame time course of development as the current flow in the conductors.The conductors may be distributed in space to produce three orthogonalgradients X, Y, Z. Each of the gradients may be independently pulsed bya separate time dependent current waveform. When the set of conductorsis in close proximity to an electrically conductive object, such as theferromagnetic pole of a primary field magnet assembly, pulsing magneticfield gradients will in turn generate currents and their associatedmagnetic fields in the electrically conductive object. These secondarymagnetic fields oppose the establishment of the gradient magneticfields. Such eddy currents thus create a delay in the establishment ofsteady state levels of the magnetic field gradients.

A typical imaging procedure involves the use of three orthogonalmagnetic field gradients, Z, X and Y, which are pulsed coordinatelyalong with bursts of radio frequency energy. An example of this is asfollows: the Z gradient is pulsed on for two brief time periods duringwhich a 90° radio frequency pulse in the first time period and a 180°radio frequency pulse in the second time period are used to select aslice of anatomy of interest and to induce the nuclear spin systemwithin that slice to generate an NMR signal. Once the slice is selectedby the Z gradient, the two remaining orthogonal gradients are used toconfer spatial encoding on the NMR signal in the two orthogonaldirections. Thus, the Y-gradient will encode on the basis of phaseadvances imparted on a series of signal responses by using a pulsedgradient waveform of progressively increasing area; and the X gradient,which is pulsed on during the signal collection period, will frequencyencode the NMR signal in the third orthogonal direction.

The NMR signal will be processed to yield images which give an accuraterepresentation of the anatomical features in the selected slice, as wellas provide excellent soft tissue contrast. NMR signals may be processedusing various algorithms depending upon the precise nature of the dataacquisition procedure; however, all methods employed rely on the abilityto spatially encode the signal information by making use of magneticfield gradients which are time modulated and sequentially pulsed invarious modes to effect the desired result.

Since most of the aspects of a pulse sequence, such as radio frequencyexcitation of the nuclear spin system, and acquisition of spatiallyencoded information are predicated on the existence of stable, steadystate magnetic field gradients, the existence of eddy currents lengthensthe time course of the pulse sequence, and thus the imaging process.Also, eddy currents inhibit the ability to follow a faster imagingregimen which might yield potentially more valuable diagnosticinformation. A reduction or suppression in eddy currents is therefore adesirable goal in NMR imaging and is the subject of this invention.

The pulsed gradient sequence described above is by no means anexhaustive treatment of pulse sequences used in magnetic resonanceimaging. More complete descriptions of pulse sequences and how they arevaried to yield medically diagnostic information may be found innumerous publications, for example "Magnetic Resonance Imaging" bySTARK, DAVID, D., and BRADLEY Jr., WILLIAM, G., (C.V. MOSBY COMPANY,1988).

A traditional method used to overcome eddy currents is to overdrive thegradient voltage waveform during the early phases of establishment ofdesired magnetic field gradient levels. This method has proven effectivein suppressing the effects of eddy currents. However, the method stilllimits the execution of pulse sequences with more rapidly switchingmagnetic field gradients, and must be tailored to the amplitude andduration of the individual waveforms. Furthermore, such methods do notaddress the problem of spatial dependence of the eddy currents, leavingonly best compromise solutions to cope with the variations in eddycurrent magnitude and duration across the imaging region of interest.

SUMMARY OF THE INVENTION

The present invention addresses these shortcomings and provides a meansto effectively suppress eddy currents through the use of materials whichpossess desirable magnetic permeability and electrical resistivity. Inaddition, the present invention employs highly conductive materials toenhance the signal-to-noise performance of medical NMR imaging systems.

There are several desirable properties which materials require in orderto effectively reduce the time course of eddy currents. In fact, softmagnetic materials as a class of materials generally possess theseproperties, although various soft magnetic materials will exhibit suchproperties to differing degrees and in different combinations.

In particular, the magnetic permeability and the electrical resistivityare important parameters. A high permeability will allow a particularmaterial to carry more magnetic flux than material with lesserpermeability. Once the capacity of the material to carry flux isexceeded, leakage will occur resulting in establishment of a fringemagnetic field outside the physical boundaries of the material. One ofthe major benefits of using ferromagnetic material is the relativelyhigh permeability. Alloys comprised of the ferromagnetic elements iron,cobalt and nickel are able to achieve permeability values as high as300,000 and are therefore referred to as high permeability materials.

Another materials parameter of importance to the present invention isthe electrical resistivity. Materials with higher electrical resistivityhave reduced current carrying capacity. This property serves to inhibitthe formation of eddy currents, which is clearly a desirable feature inthe present situation.

Thus, a combination of high permeability to contain the gradientmagnetic field, and high electrical resistivity to prevent the formationof eddy currents is desirable; materials possessing such properties,including soft magnetic materials, are beneficial when incorporated intothe primary field magnet assembly of a magnetic resonance imagingsystem. A number of embodiments of the present invention detailed belowtake advantage of such materials.

In addition to the desirable properties of permeability and resistivity,soft magnetic materials tend to have low coercivity. In fact, materialsexhibiting coercivity values less than 5 Oersteds are considered soft.Low coercivity materials have a generally narrower hysteresis loop whenB, magnetic induction, is plotted versus H, the magnetizing field. Sincethe loss in energy is directly related to the area of the hysteresisloop, a narrow loop implies a more efficient material in terms of itsmagnetic field properties.

High permeability materials which exhibit the desirable propertiesmentioned above function effectively in transient magnetic fields wherethe frequency components of the transient are in the audio range, i.e.less than approximately 10 KHz. These same materials however aretypically subjected to electromagnetic radiation in the radio frequencyrange during the imaging procedure. Indeed, radio frequencies used todayin magnetic resonance imaging span a broad range from approximately 1MHz to approximately 200 MHz.

Under conditions which exist in magnetic resonance imaging, highpermeability materials may couple to the radio frequency antennascausing a reduction in the Q-factor of the tuned antennas. The Q-factorof an antenna is directly related to the signal-to-noise (S/N)performance of a magnetic resonance imaging system, with S/N α√Q-factor.The desirable properties of high permeability materials as related tosuppressing eddy currents may therefore deteriorate the signal-to-noiseperformance of the magnetic resonance scanner. This effect is at leastpartially related to the proximity of the RF antennas to the highlypermeable material, with the deterioration more pronounced as theantennas approach the high permeability material. However, the presentinvention includes the novel practice of employing highly conductivematerial, which has the effect of generally decoupling the radiofrequency antennas from their environment.

The present invention details the implementation of layers of highlyconductive material of the proper thickness which restores the Q-factorof the radio frequency antennas, thereby improving signal-to-noise ratioover the situation devoid of such a highly conductive layer. Sincehighly conductive materials are also capable of conducting andperpetuating eddy currents, the proper balance needs to be achieved whencombining materials which suppress eddy currents at the expense ofreducing the radio frequency antenna performance, and materials whichpreserve the Q factor of the radio frequency antennas at the expense ofperpetuating eddy currents. This balance is achieved in the presentinvention.

High permeability materials are of many physical and chemical typesincluding ferrites, bonded metal particulates, unbonded metalparticulates, composites, metals and metal alloys. An earlier invention,commonly assigned U.S. Pat. No. 5,061,897, included some members of thisgroup, or more specifically, those with a magnetic permeability greaterthan 1000, and an electrical resistivity at least 1000 micro ohm -cm.Metals and metal alloys in general were excluded on the basis of theelectrical resistivity criteria since such materials have resistivityless than 1000 micro ohm -cm.

Additional investigation into high permeability materials whereelectrical resistivity is less than 1000 micro ohm -cm, has identified aclass of materials which effectively suppresses eddy currents. Unlikethe materials which are included under commonly assigned U.S. Pat. No.5,061,897, the materials which are the subject of the present inventioninclude metals and metal alloys which possess specific characteristicsas related to initial permeability and electrical resistivity. Suchmaterials are herein referred to as eddy current suppressing materials.

This invention includes metal alloys comprising nickel, iron, cobalt,chromium, molybdenum, copper, and aluminum, in a solid homogeneous form,in contrast to the same metal alloys in a particulate composite formwhich was an embodiment of the earlier invention. In addition, thepresent invention includes embodiments which combine the use of highpermeability materials in general with highly conductive materials; andembodiments for the general implementation of highly conductivematerials to decouple the radio frequency antennas from the environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: A diagram of a typical pulse sequence used in magnetic resonanceimaging.

FIG. 2: A cross-sectional view of an iron core magnet.

FIG. 3: A cross-sectional view of a ferromagnetic pole including athickness of eddy current suppressing material, and schematicrepresentation of subassemblies of a magnetic resonance imaging system.

FIG. 4: A cross-sectional view of a ferromagnetic pole including amultiple layer thickness of high permeability material.

FIG. 5: A cross-sectional view of a ferromagnetic pole including athickness of high permeability material and a layer of highly conductivematerial.

FIG. 6: A cross-sectional view of a ferromagnetic pole with highlyconductive material layer, a set of conductors, (schematicallyrepresented) and a thickness of high permeability material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention comprises a primary field magnet assembly, of thetype used in medical magnetic resonance imaging systems, which has beendesigned with the capability to suppress eddy currents during magneticresonance imaging through the use of eddy current suppressing material.In addition, this invention includes the use of high conductivitymaterial to decouple the radio frequency antennas from theirenvironment, and to preserve the signal-to-noise performance of theimaging system. Such high conductivity material is highly effective whenemployed in combination with a primary field magnet assembly asdescribed herein.

The importance of suppressing eddy currents in conductive magneticresonance imaging can be seen by examining FIG. 1 which shows asimplified, but typical, pulse sequence. Three orthogonal magnetic fieldgradient waveforms, labelled Z, X and Y, define the spatial encoding ofthe anatomical region of interest to be imaged. As represented in FIG.1, the Z-gradient waveform defines the slice select function and willdetermine the thickness of the anatomical slice to be imaged. Theplateau portions of the waveform, 1 and 2, correspond to the periods oftime in the pulse sequence where respectively, the 90° 3, and 180° 4,radio frequency pulses, which are used to excite the nuclear spinsystem, take place. Once the slice thickness has been defined,resolution in the two orthogonal directions is determined by the X- andY-gradients. For example, in FIG. 1, the X-gradient performs the readout function in the pulse sequence and has achieved a plateau 5, duringthe data collection of the NMR signal 6. The Y-gradient 7, functions asa phase encoding gradient, and advances the phase of the NMR signal byprogressively increasing the amplitude, on repetitions of the pulsesequence. The image data are formed from a collection of numerous NMRsignal 6, where each signal is spatially encoded differently through anadvance of the phase encoding gradient 7.

It is to be appreciated that each of the switching magnetic fieldgradients must achieve the desired levels of magnetic field strength ina timely manner as represented in FIG. 1, in order to functionallycoordinate with the other magnetic field gradients, the RF pulses andthe collection of the NMR signal. Eddy currents retard the establishmentof desired levels of the magnetic field gradient and therefore limit thespeed of the pulse sequence.

The designation of the X-, Y- and Z-gradients in this illustration isarbitrary, and in common practice any of the three orthogonal gradients,or combinations thereof, can function as a slice select gradient,readout gradient, or phase encode gradient. Therefore, when gradientsare employed in any such manner, it is to be considered within the scopeof this invention. Additionally, the development of imaging technologyhas resulted in more complicated gradient waveforms and pulse sequences.The pulse sequence in FIG. 1 is merely illustrative and not meant tolimit the scope of this invention.

As magnetic resonance imaging technology has improved, there has been atrend toward faster pulse sequences. Magnetic field gradient switchingtimes on the order of hundreds of microseconds are desirable.Furthermore, shorter switching times have been accompanied by the needto achieve greater gradient amplitudes concomitant with the desire forimproved spatial resolution in the images. These developments have allcontributed to the need to improve eddy current control.

One embodiment of the present invention is shown in FIG. 2 whichcontains elements of the primary field magnet assembly shown incross-section. The assembly has two vertical members 10, and twohorizontal members 11, which are connected in each of the four cornersby a pair of bolts, 12. Connected to each of the two horizontal members11, and centrally located thereon is a cylindrical member 13, surroundedby a concentric annular ring, 14. The vertical members 10, horizontalmembers 11, connecting bolts 12, cylindrical members, 13 and annularrings, 14 are comprised of ferromagnetic material, and taken together,comprise the ferromagnetic structure of the primary field magnetassembly.

Also shown in FIG. 2 is the means for generating magnetic flux, 15. Themagnetic flux generating means, 15, is disposed in the region betweenthe vertical members 10, and the annular ring 14. The magnetic fluxgenerating means essentially surrounds the annular ring 14, and is inclose proximity to both the annular ring 14, and the horizontal member11. An essential feature of the primary field magnet assembly is theefficient coupling between the ferromagnetic structure and the magneticflux generating means. The stronger this coupling, the greater will bethe proportion of magnetic flux carried by the ferromagnetic structureand consequently the higher will be the magnetic field value as measuredin the magnet gap 18. The magnetic flux generating means may becomprised of permanent magnet material, resistive electromagneticwindings, or superconductive windings.

Taken together, the cylindrical member 13, and annular ring 14constitute the support structure for each of the pole pieces 16a and16b. The two pole pieces, 16a and 16b, are arranged in opposed fashion,with each pole approximately equidistant from an imaginary horizontalline, 17, passing midway through the primary field magnet assembly, andwith the centers of each pole aligned vertically with each other. Eachpole is comprised of ferromagnetic material and is attached to thesupport structure for maximum coupling.

The separation of the two poles defines a region of space commonlycalled the magnet gap 18, into which the anatomical region of interestof a patient is placed preparatory to a magnetic resonance imagingprocedure. This magnet gap also defines an area containing a highlyuniform magnetic field, where the magnetic flux originates in one pole,for example 16a, and terminates in the other pole, for example 16b. Themagnetic flux passes through the pole piece and into the ferromagneticstructure, which provides a return path for the magnetic flux to thepole piece, 16a, which was designated as the point of origin formagnetic flux in this discussion. It should be clear that this exampleis not meant to exclude the possibility of the direction of the magneticfield being opposite to what has been described. The direction of themagnetic field is easily chosen by one skilled in the art.

In providing a return path for the magnetic flux which crosses themagnet gap, the ferromagnetic structure also directs and generallycontains the magnet flux. More efficient primary field magnet assembliesmay be constructed by increased coupling between the ferromagneticstructure and the ferromagnetic poles. The embodiment shown herein hasthe ferromagnetic structure contiguous with the ferromagnetic poles, andis indicative of a high degree of magnetic coupling.

As shown in FIG. 2, the portion of the region of space which is in closeproximity to the ferromagnetic poles contains additional subassembliessuch as a shim bar 19. A more detailed description of the structuralfeatures of the subassemblies and their relationship to theferromagnetic pole may be seen in a cross-section view in FIG. 3. In thepreferred embodiment, the ferromagnetic pole 16, and shim bar 19, arecircular. A thickness of eddy current suppressing material 21, is placedproximate and generally coextensive with, the ferromagnetic pole, 16.Subassemblies, in addition to the shim bar 19, which are necessary forconducting magnetic resonance imaging procedures are placed in themagnet gap. The additional subassemblies include, but are not limitedto, radio frequency antennas for transmitting 22, and receiving 23,radio frequency energy during the imaging procedure; and a set ofconductors 24 which, when arranged in the proper spatial relationship toone another, are capable of generating the three orthogonal magneticfield gradients, X, Y and Z. As schematically represented in FIG. 3, theset of conductors 24, is arranged in a triple layer structure placed inthe magnet gap, where each layer represents a grouping of conductorscapable of generating one of the three orthogonal gradients, X, Y or Z.Such additions to the primary field magnet assembly are generallytypical of modern day NMR medical imaging systems.

The thickness of eddy current suppressing material 21, has magnetic andelectrical properties which enable it to effectively suppress eddycurrents. The initial magnetic permeability of such materials is greaterthan 1000. When the materials are subjected to increasing levels ofexternal magnetic field, the magnetic permeability decreasesprecipitously at a point where the material becomes saturated.Sufficiently high values of permeability are necessary for the presentinvention in order to provide a means to contain the magnetic fieldsgenerated by the pulsed gradients during magnetic resonance imaging. Inaddition, however, the values of magnetic field at which saturationoccurs for the eddy current suppressing material must be sufficientlyhigh in order to permit such materials to operate effectively atmagnetic field strength values of the primary field magnet.

For the eddy current suppressing materials to be effective, they mustpossess sufficiently high electrical resistivity to suppress theformation of eddy currents which are generated in such materials by thepulsing magnetic fields. Some materials with electrical resistivity lessthan 1000 micro ohm -cm are effective in significantly reducing eddycurrents.

Eddy current suppressing materials with the combined properties ofinitial magnetic permeability greater than 1000, and electricalresistivity less than 1000 micro ohm -cm represent a class of materialscapable of containing the magnetic flux generated by the pulsed magneticfield gradients and suppressing eddy currents which are generated bythis process.

Eddy current suppressing materials which exhibit the combination ofdesirable properties of initial permeability and electrical resistivityinclude metals and metal alloys. Metals and metal alloys exhibit thephysical characteristics of being solid and homogeneous. As used herein,a solid is defined as a material which has been made by any commonlyemployed metallurgical technique yielding a finished material which ismetallurgically virtually one hundred percent dense. Also, as usedherein, the term homogeneous refers to a material whose individualgrains have the same physical, electrical and magnetic properties asdoes the identical material in bulk form. It is to be understood thatthe individual grains of the material are comprised of allmicrostructural components and characteristics which are common to thebulk material as a whole.

One type of metal alloy from the group of eddy current suppressingmaterials which exhibits the desirable properties of initialpermeability and electrical resistivity is the nickel-iron alloys.Nickel-iron alloys vary in nickel content from as low as 35% (n.b., allpercentages recited herein are on the basis of weight) to as much as80%. The remaining alloy percentage is primarily comprised of iron, withdifferent nickel-iron alloys containing smaller amounts of otherelements. Examples of other elements include, but are not limited to,molybdenum, silicon, manganese, carbon, copper and chromium.

Varying the weight percentage composition of an eddy current suppressingmaterial predominantly comprised of nickel and iron changes theoperating characteristics of the material. To illustrate, commerciallyavailable material comprising 80% nickel has an initial permeability off35,000, a maximum permeability of 350,000, and will become saturated at8,200 gauss; whereas, a material comprising 48% of nickel has an initialpermeability of 11,000, a maximum permeability of 100,000, and saturatesat 15,200 gauss. Variations in the precise nickel content of the eddycurrent suppressing material chosen will be determined by the details ofthe intended use. However, it should be clear that any use ofnickel-iron alloy materials in particular, and eddy current suppressingmaterials in general, with the combined properties of initialpermeability greater than 1000 and electrical resistivity less than 1000micro ohm -cm constitutes practice of the present invention.

A primary field magnet assembly including a ferromagnetic pole whichemploys eddy current suppressing material disposed on the pole andcomprising 80% nickel is the preferred embodiment of this invention.Such materials are commercially available under various trade names,including, as an example, "Permalloy 80."

The thickness of eddy current suppressing material may be implemented asa single layer 21, as shown in FIG. 3, or consisting of more than onelayer. In the latter case, the desired thickness is achieved byconstructing a stacked package of multiple layers of material. As shownin FIG. 4, each layer of eddy current suppressing material 25 iselectrically isolated from neighboring layers by a layer of insulatingmaterial 26. The thickness of the stacked package of eddy currentsuppressing material is thus composed of a multiplicity of alternatinglayers of the eddy current suppressing material and the electricallyinsulating material. Alternatively, the stacked package of multiplelayers of eddy current suppressing material may be constructed from amultiplicity of layers without the layers of electrically insulatingmaterial interleaved between them. Each of these alternatives toachieving a total thickness of material necessary to effectivelysuppress eddy currents is an alternative embodiment of the presentinvention.

A subject of commonly assigned U.S. Pat. No. 5,061,897 was highpermeability materials including ferrites, bonded particulate metals,unbonded particulate metals, composites incorporating particulatemetals, and combinations thereof. These materials as a group, arecharacterized by relatively high electrical resistivity, generallygreater than 1000 micro ohm -cm. In contrast, the present inventionincorporates materials with electrical resistivity less than 1000 microohm -cm which also effectively suppress eddy currents, and are usefulwhen incorporated into the primary field magnet assembly of NMR medicalscanners.

The placement of radio frequency antennas in the proximity of highpermeability materials, such as eddy current suppressing material, orthose which are the subject of commonly assigned U.S. Pat. No.5,061,897, can result in coupling between the radio frequency antennasand the highly permeable materials. Such coupling reduces the Q-factorof the RF antennas and results in a loss in signal-to-noise during theimaging procedure. The inclusion of a relatively thin layer of highlyconductive material between the radio frequency antennas and the highlypermeable material employed to suppress eddy currents decouples theradio frequency antennas and restores the Q-factor of these antennas,resulting in the restoration and preservation of signal-to-noise. Sincehighly conductive materials are capable of sustaining eddy currents,only thin layers can be tolerated so as not to counteract the eddycurrent suppressing material characteristics of the highly permeablematerials. Thus, highly conductive materials may be employed incombination with high permeability materials as shown in FIG. 5. In thepreferred embodiment, a sheet of highly conductive material 27, is addedin a fashion overlying and approximately coextensive with, the thicknessoff eddy current suppressing material, and beneath the shim bar ring.The highly conductive material in this embodiment, is aluminum, having athickness of approximately 0.0003 inches. Highly conductive layers ofaluminum with a thickness of approximately 0.0005 inches have provenslightly more effective at improving the Q-factor of the radio frequencyantennas, with a concomitantly small additional reduction of theeffectiveness of the eddy current suppressing material.

There are several variations of the preferred embodiment. Other highlyconductive materials from a group including copper, silver, and gold, aswell as aluminum, or combinations thereof, may be used. Also, adifferent thickness of material may be desirable, depending upon thehighly conductive material selected. Moreover, the layer of highlyconductive material need only be placed between the highly permeablematerial and the radio frequency antennas and not necessarily in closeproximity to the highly permeable material. This is illustrated in FIG.6 which shows the layer of highly conductive material 27, placedoverlying subassemblies such as the set of conductors 24, and shim bar19. It should be understood that these alternative embodimentsconstitute practice of the present invention.

In addition to using highly conductive materials to decouple radiofrequency antennas from highly permeable materials in NMR medicalimagers, employing highly conductive material is beneficial as a meansfor decoupling radio frequency antennas generally from environmentalfactors which reduce signal-to-noise performance by coupling with the RFantennas. As an example, large antennas, or antennas placed in proximityto elements of the primary field magnet assembly, may perform poorly dueto coupling between the antennas and such elements. These elementsinclude, but are not limited to shim bars, or portions of theferromagnetic structure. Placement of a thin sheet of highly conductivematerial between the RF antennas and these elements will enhance thesignal-to-noise performance of the imaging system. Thus, the use ofhighly conductive material to decouple RF antennas from environmentalfactors in general is an embodiment of this invention and is independentof the presence of eddy current suppressing highly permeable material inthe primary field magnet assembly.

Another embodiment of the present invention involves upgrading a medicalNMR scanning apparatus to improve the eddy current responsecharacteristics.

The existence of subassemblies in close proximity to the ferromagneticpole as is shown in FIG. 3 may obscure the access required toincorporate the high permeability magnetic material, or highconductivity material in situations, for example, where a magneticresonance imaging apparatus has already been constructed. Such asituation will require providing access to the pole surface, and mayinvolve separating subassemblies from the ferromagnetic polesufficiently to allow placement of the high permeability magneticmaterial, high conductivity material, or both. Following these placementsteps, either the same subassemblies as were originally separated fromthe proximity of the ferromagnetic pole surface, or differentsubassemblies, will be secured such that the final configuration of theprimary field magnet assembly, ferromagnetic pole and subassemblies arecapable of performing magnetic resonance imaging.

As alternative embodiments of this upgrade method, it is possible toalter the steps of this method and still achieve a desirable result. Forexample, the step of placing the layer of high conductivity material forpreventing radio frequency absorption could be conducted after the stepof securing at least some of the subassemblies into said primary fieldmagnet assembly. This method would permit, for example, securing the setof conductors which are used to generate the magnetic field gradients,prior to placement of the layer of highly conductive material.Additionally, in situations where the high permeability magneticmaterial is not necessary or desirable, the method of upgrading couldstill be conducted while eliminating the step of placing a thickness ofhigh permeability magnetic material.

Eddy currents can only exist in electrically conductive materials. Thus,all elements of the primary field magnetic assembly, in addition to theferromagnetic pole, which are electrically conductive are capable ofsupporting eddy currents. In yet another embodiment, highly permeablematerials may therefore be employed generally to reduce eddy currents inelectrically conductive portions of the magnet assembly. Such highlypermeable materials include eddy current suppressing materials, andmaterials covered in commonly assigned U.S. Pat. No. 5,061,897.

This embodiment is illustrated in FIG. 2, where for example, a thicknessof highly permeable material, 8, is used to cover portions of theelectrically conductive ferromagnetic vertical member 10, of the primaryfield magnet assembly. The thickness of highly permeable material 8, maybe a single layer, or a multiplicity of layers where each layer iselectrically insulated from other layers, or where the layers of highlypermeable material are electrically conductive with each other. Theconstruction of the multiple layer thickness is the same as describedearlier with reference to FIG. 4. This embodiment may be equally wellpracticed on any electrically conductive elements of the primary fieldmagnet assembly.

In another embodiment shown in FIG. 2, a layer of highly conductivematerial, 9, may also be incorporated overlying electrically conductiveportions of the magnet assembly to decouple the radio frequency antennasand improve their signal-to-noise performance. The practice of thisembodiment is beneficial when the highly conductive material, 9, isimplemented proximate to, and approximately co-extensive with the highpermeability magnet material, 8, used to suppress eddy currents.However, implementation of highly conductive material in general todecouple radio frequency antennas from electrically conductive portionsof the magnet assembly even in cases where high permeability magnetmaterial is not employed, is also beneficial and is an embodiment ofthis invention.

We claim:
 1. A primary field magnet assembly for use in medical NMR imaging systems, comprising:a) a ferromagnetic structure which provides a return path for magnetic flux, b) means for generating said magnetic flux, said means magnetically coupled to said ferromagnetic structure to establish magnetic flux flowing through said ferromagnetic structure, c) two opposed ferromagnetic poles spaced apart from one another and forming a gap therebetween for receiving a portion of human anatomy therein, said ferromagnetic poles being magnetically coupled to said ferromagnetic structure to develop a magnetic field between said poles, d) a layer of high permeability magnetic material, said material providing means to reduce eddy currents in said ferromagnetic poles, and said material placed proximate to a surface of each said pole which faces said gap, and e) means for preventing radio frequency absorption by said high permeability magnetic material, said means overlying said high permeability magnetic material for each of said ferromagnetic poles.
 2. The primary field magnet assembly as described in claim 1, wherein said means for generating magnetic flux comprises permanent magnetic material.
 3. The primary field magnet assembly as described in claim 1, wherein said means for generating magnetic flux comprises resistive electromagnetic windings.
 4. The primary field magnet assembly as described in claim 1, wherein said means for generating magnetic flux comprises superconductive windings.
 5. The primary field magnet assembly as described in claim 1, wherein said high permeability material comprises eddy current suppressing material having an initial permeability greater than approximately 1000 and a resistivity less than 1000 micro ohm-cm.
 6. The primary field magnet assembly as described in claim 1, wherein said high permeability magnetic material is a nickel-iron alloy comprising at least 35% nickel.
 7. The primary field magnet assembly as described in claim 1, wherein said high permeability magnetic material is a nickel-iron alloy comprising approximately 80% nickel, approximately 15% iron, and the remaining approximately 5% comprising at least one element of the group consisting of molybdenum, silicon, manganese, copper and carbon.
 8. The primary field magnet assembly as described in claim 7, wherein said high permeability magnetic material is a nickel-iron alloy comprising about 80% nickel, 14.93% iron, 4.2% molybdenum, 0.35% silicon, 0.5% manganese, and 0.02% carbon.
 9. The primary field magnet assembly as described in claim 1, wherein said high permeability magnetic material is selected from a group consisting of ferrites, bonded particulate metals, unbonded particulate metals, composites incorporating particulate metals and combinations thereof.
 10. The primary field magnet assembly as described in claim 1, wherein said high permeability magnetic material comprises a single layer.
 11. The primary field magnet assembly as described in claim 1, wherein said high permeability magnetic material comprises multiple layers.
 12. The primary field magnet assembly as described in claim 11, wherein each layer of said multiple layers of high permeability magnetic material is electrically isolated from other layers of said multiple layers.
 13. The primary field magnet assembly as described in claim 1, further comprising a set of conductors for generating magnetic field gradients for conducting magnetic resonance imaging, said set of conductors placed between said high permeability magnetic material and said means for preventing radio frequency absorption, for each of said poles.
 14. The primary field magnet assembly as described in claim 1, further comprising a set of conductors for generating magnetic field gradients for conducting magnetic resonance imaging, said set of conductors placed overlying said means for preventing radio frequency absorption, for each of said poles.
 15. The primary field magnet assembly as described in claim 1, wherein said means of preventing radio frequency absorption comprises high electrical conductivity material.
 16. The primary field magnet assembly as described in claim 1, wherein said means of preventing radio frequency absorption comprises at least one element of the group consisting of aluminum, copper, silver and gold.
 17. The primary field magnet assembly as described in claim 1, wherein said means of preventing radio frequency absorption has a thickness of approximately 0.0003 inches.
 18. The primary field magnet assembly as described in claim 1, wherein said layer of high permeability material is in a substantially homogeneous solid form.
 19. A primary field magnet assembly for use in medical NMR imaging systems, comprising:a) a ferromagnetic structure which provides a return path for magnetic flux, b) means for generating said magnetic flux, said means magnetically coupled to said ferromagnetic structure to establish magnetic flux flowing through said ferromagnetic structure, c) two opposed ferromagnetic poles spaced apart from one another and forming a gap therebetween for receiving a portion of human anatomy therein, said ferromagnetic poles being magnetically coupled to said ferromagnetic structure to develop a magnetic field between said poles, (d) a set of conductors overlying each of said ferromagnetic poles, each of said set of conductors for generating magnetic field gradients, and e) means for preventing radio frequency absorption by said ferromagnetic poles, said means overlying each of said set of conductors.
 20. The primary field magnet assembly as described in claim 19, wherein said means for generating magnetic flux comprises permanent magnetic material.
 21. The primary field magnet assembly as described in claim 19, wherein said means for generating magnetic flux comprises resistive electromagnetic windings.
 22. The primary field magnet assembly as described in claim 19, wherein said means for generating magnetic flux comprises superconductive windings.
 23. The primary field magnet assembly as described in claim 19, wherein said means for preventing radio frequency absorption comprises high electrical conductivity material.
 24. The primary field magnet assembly as described in claim 19, wherein said means for preventing radio frequency absorption comprises at least one element of the group consisting of aluminum, copper, silver and gold.
 25. The primary field magnet assembly as described in claim 19, wherein said means for preventing radio frequency absorption has a thickness of approximately 0.0003 inches. 